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BOSTON -- A former associate of James "Whitey" Bulger testified Thursday that he and the alleged criminal mastermind of a South Boston gang were FBI informants for about 15 years.

Testifying in Bulger's federal racketeering trial, Stephen "The Rifleman" Flemmi stared down Bulger, who he hadn't seen since 1994. Flemmi, serving a life sentence after pleading to 10 murders, is considered a key witness in the case against Bulger, the 83-year-old notorious leader of South Boston's Winter Hill gang.

As he was led into the witness box, Flemmi gave Bulger a long, hard stare, and Bulger glanced over at him. Afterward, Flemmi, who's now 79, stood in the box with his hands on his hips and stared confidently at Bulger. The two exchanged words briefly, though it wasn't clear what they said to each other.

Bulger has already had two profanity-laced outbursts during the trial, one directed at former protege Kevin Weeks; the other at a former FBI agent who admitted taking payoffs from Bulger.

Though Flemmi's first appearance was short, prosecutors worked quickly, asking Flemmi to explain how he and Bulger gave information about the Mafia and other criminal organizations to an FBI agent. Flemmi said the two were top-echelon government informants from about 1975 to 1990, a claim the defense has repeatedly tried to rebut.

Flemmi's testimony - which continues Friday - came after the body of Stephen "Stippo" Rakes was found near Lincoln, Mass. Rakes had been scheduled to testify that Bulger and members of his Winter Hill gang forced him at gunpoint to sell his liquor store to Bulger in 1984. But the U.S. Attorney's Office told Rakes Tuesday that they did not plan to call him to testify, according to Rakes' family.

Rakes' body was found at about 1:30 p.m. Wednesday, according to the Middlesex District Attorney's Office. There were no signs of trauma to Rakes' body. The medical examiner's office is conducting an autopsy to determine the cause and manner of death.

ABC News reported that police told Rakes' family that the death appeared to be a suicide. No phone or wallet was found on the body. Local and state authorities are investigating.

Steve Davis, a Rakes' friend and brother of another alleged Bulger victim, told the Boston Globe that the decision not to testify had devastated Rakes, 59.

Davis told ABC News that Rakes would not have killed himself and "was looking forward to testifying." He said Rakes, who had been attending Bulger's trial regularly over the past six weeks, had planned to deliver a "big bombshell" on the witness stand.

Rakes was a particularly angry and determined victim of Bulger's gangland tactics.

He was apparently supposed to testify that Bulger, 83 and Flemmi threatened his daughters at gunpoint and forced him to sell his South Boston liquor store for $100,000. The building eventually became Bulger's headquarters.

Rakes was an outspoken pre-trial critic of Bulger. "The day I see him in a box, not breathing, will be better," Rakes said in April.

Tommy Donahue, son of alleged Bulger victim Michael Donahue, said Thursday that Rakes "wanted to get up there and tell his side of the story."

Among relatives of Bulger's victims, word of Rakes' death was devastating. "I feel so bad,'' Patricia Donahue, whose husband was allegedly slain by Bulger, told the Boston Globe. "You sit here every day with these victims and they become part of your life. He seemed so spirited. He had a lot of spirit."

Bulger, a much-feared South Boston gangster for decades, fled the city in 1994 ahead of his arrest. He was captured in California two years ago after 16 years on the run.

Trial observers say Rakes' death shouldn't affect the outcome of the trial. "He had a special level of outrage and really wanted to tell his story, but his story was not consistent" with Weeks' prior testimony, says Rosanna Cavallaro, a Suffolk University criminal law professor.

Last week, the testimony of Weeks, Bulger's former right-hand man, included an account of how Bulger acquired Rakes' liquor store. Weeks denied that the gang forced Rakes to sell the store, saying Rakes had agreed to an offer from Bulger to buy the store for $100,000.

He said when they arrived at Rakes' house to close the deal, Rakes said his wife didn't want to sell the store and complained about the price.

"He was trying to shake us down," Weeks testified.

Weeks said he pulled a gun out of his waistband and put it on a table, in front of Rakes' two young daughters, who were in the room. One of the girls was bouncing on Bulger's lap and reached for the gun, and Bulger told Weeks to put it away.

Bulger told Rakes that he couldn't back out of the sale and they made the deal, according to the testimony.

Rakes was present for the testimony and later disputed the account, saying he was forced to sell the liquor store.

"Kevin continues to lie, as usual, because that's what he has to do," Rakes said that day. "My liquor store was never for sale - never, never, never."